From e38938b332ca751dfc5e784f242d620016e8ca43 Mon Sep 17 00:00:00 2001
From: Stan Hu <stanhu@gmail.com>
Date: Tue, 15 May 2018 23:06:55 -0700
Subject: Fix Error 500 viewing admin page due to statement timeouts

Uses PostgreSQL tuple estimates to provide a much faster yet approximate
count. See https://wiki.postgresql.org/wiki/Slow_Counting for more details.
We only use this fast method if the table has been analyzed or vacuumed
within the last hour.

Closes #46255
---
 lib/gitlab/database/count.rb | 48 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 lib/gitlab/database/count.rb

(limited to 'lib')

diff --git a/lib/gitlab/database/count.rb b/lib/gitlab/database/count.rb
new file mode 100644
index 00000000000..3374203960e
--- /dev/null
+++ b/lib/gitlab/database/count.rb
@@ -0,0 +1,48 @@
+# For large tables, PostgreSQL can take a long time to count rows due to MVCC.
+# We can optimize this by using the reltuples count as described in https://wiki.postgresql.org/wiki/Slow_Counting.
+module Gitlab
+  module Database
+    module Count
+      CONNECTION_ERRORS =
+        if defined?(PG)
+          [
+            ActionView::Template::Error,
+            ActiveRecord::StatementInvalid,
+            PG::Error
+          ].freeze
+        else
+          [
+            ActionView::Template::Error,
+            ActiveRecord::StatementInvalid
+          ].freeze
+        end
+
+      def self.approximate_count(model)
+        return model.count unless Gitlab::Database.postgresql?
+
+        execute_estimate_if_updated_recently(model) || model.count
+      end
+
+      def self.execute_estimate_if_updated_recently(model)
+        ActiveRecord::Base.connection.select_value(postgresql_estimate_query(model)).to_i if reltuples_updated_recently?(model)
+      rescue *CONNECTION_ERRORS
+      end
+
+      def self.reltuples_updated_recently?(model)
+        time = "to_timestamp(#{1.hour.ago.to_i})"
+        query = <<~SQL
+          SELECT 1 FROM pg_stat_user_tables WHERE relname = '#{model.table_name}' AND
+          (last_vacuum > #{time} OR last_autovacuum > #{time} OR last_analyze > #{time} OR last_autoanalyze > #{time})
+        SQL
+
+        ActiveRecord::Base.connection.select_all(query).count > 0
+      rescue *CONNECTION_ERRORS
+        false
+      end
+
+      def self.postgresql_estimate_query(model)
+        "SELECT reltuples::bigint AS estimate FROM pg_class where relname = '#{model.table_name}'"
+      end
+    end
+  end
+end
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